Date of Award

Spring 1997

Project Type

Dissertation

Program or Major

Engineering

Degree Name

Doctor of Philosophy

First Advisor

Todd Gross

Abstract

An experimental process was developed to study the effects of material properties on fracture surface interference in Mode II loaded cracks in 7075 aluminum. 4340 and 8642 steels. Factors studied included elastic modulus, yield strength, mode mixity and fracture surface geometry. An asymmetric four point bend specimen and fixture was developed to apply pure Mode II and mixed Mode I and II loads to edge cracks. Three dimensional displacement fields were measured as a function of applied load using an electro-optical holographic interferometric facility. Stress intensity factors were obtained by fitting observed 2-D field displacements to basis functions derived from analytical field equations in a multi-variable regression. In addition, an experimental characterization of fracture surface roughness and a numerical study of fracture interaction was reported.

The resistance Mode II stress intensity factor was found to exhibit a yield strength dependence while an inverse elastic modulus effect was observed. The magnitude of the K$\sb{\rm llres}$ was found to have a direct dependence on the applied mode mixity. The lock up mode of interaction behavior implied by the measured crack face displacements and the estimated SIF was yield strength and elastic modulus dependent with lock up observed prior to sliding or smearing where normalized yield strength, ${{\sp{\sigma}\rm ys}\over{E}} > 0.006$ and not observed where ${{\sp\sigma{\rm ys}}\over{E}} < 0.002.$ It was proposed that the distinctly different behaviors observed in identical specimens were due in part to differences in the state of contact between opposing fracture surfaces.

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